A problem in bonding to nickel-titanium alloys such as Nitinol is that as the alloy is heated, desirable properties such as shape memory or superelasticity are commonly destroyed or severely degraded as the alloy is heated through its annealing temperature range. The loss of these desirable properties is also a function of the size and shape of the alloy member, as well as the time period and temperature of the applied heat. In medical applications, nickel-titanium alloy wires are commonly used to form baskets, filters, and the like that are percutaneously introduced into the patient with an introducer catheter or sheath. When the nickel-titanium wire is welded or brazed, the shape memory or superelastic properties are typically destroyed or severely degraded. As a result, a short length of stainless steel cannula is typically crimped to attach a nickel-titanium wire to itself or another device component, which undesirably stiffens the overall device and contributes to its overall bulk.
As suggested, one prior art technique for bonding two or more metallic members together is welding, in which the members are heated to their melting points and deformed. The annealing temperature of nickel-titanium alloys such as Nitinol ranges from 350 degrees to 950 degrees Fahrenheit depending on the time period that a particular temperature is applied. Since the annealing temperature range is well below the typical 2350 degree Fahrenheit melting temperature of Nitinol, welding nickel-titanium alloy wires clearly destroys the desirable shape memory and superelastic properties of the alloy particularly at the weld joint.
Another prior art technique for bonding two or more metallic members is brazing, in which a bonding material is typically heated above 425 degrees Celsius (797 degrees Fahrenheit) but below the melting temperature of the metallic members. However, brazing normally exceeds the annealing temperatures of nickel-titanium alloys, and as a result, desirable shape memory and superelastic properties are, again, typically destroyed or severely degraded.
Another bonding technique that utilizes even lower temperatures is soldering. Soldering utilizes a solder material typically having a melting point below 425 degrees Celsius. The melting point of the solder material is a function of the proportional weights of the constituent metals. Although melting temperatures of solder materials are somewhat lower, soldering may also destroy or severely degrade desirable characteristic properties of the nickel-titanium alloy member if the activation temperature of the soldering flux and the melting temperature of the solder material are too high.
Another problem with soldering to nickel-titanium alloys such as Nitinol is that these alloys readily form an outer layer of titanium oxide which prevents fluxes from wetting and solder material from flowing on the surface to form a good metallic bond or joint. The oxidation also contaminates and weakens the joint. One solution to the oxidation layer problem is the addition of one or more layers of an interface material for adhering both to the nickel-titanium alloy member surface and the bonding material. Several techniques for depositing these interface layers include electroplating or applying compounds, solutions, powders, or fluxes to the nickel-titanium alloy member surface. However, many of these techniques also present the same previously described heat problem. Another problem with adding these interface layers is that the plated surface may not have the same desirable characteristic properties as the nickel-titanium alloy member. In the case of nitinol, a nickel-plated surface exhibits local loss of flexibility and increased tendency to crack or flake when the member is flexed.
Prior to applying a material or adding interface layers to the surface of a nickel-titanium alloy member, contaminants such as titanium oxide must be removed. One solution for removing contaminants is the application of a cleaning fluid or flux. Here again, the flux must have an activation temperature lower than a particular annealing temperature of the nickel-titanium alloy for preventing destruction and degradation of desirable characteristic properties. Traditional fluxes that remove the titanium oxide from the surface of nickel-titanium alloys have activation temperatures typically above the nominal annealing temperature of the alloy. Again, the use of these fluxes destroys or severely degrades the superelasticity or shape memory properties of the alloy as the nominal annealing temperature is approached. Other mechanical solutions for removing contamination are sanding, grinding, scraping, or applying an abrasive paste to the surface of the alloy. Ultrasonic cleaning is also available to shake oxidation particles free from the alloy surface. Physically removing oxidation, however, typically leaves a residue to contaminate the metallic bond.
Still another problem with nickel-titanium alloys is that a clean surface oxidizes rapidly, even within fractions of a second when exposed to air. To prevent recurrent oxidation and contamination, gas atmosphere reduction may be utilized in which a nonoxidizing or inert gas is provided during the bonding process. Of course, the contaminants must also be removed in a nonoxidizing environment. A disadvantage of this technique is the use of an evacuation or vacuum chamber.